1. Field of the Invention
The present invention relates to an apparatus for protecting devices connected to AC power lines, telephone lines and/or networks which carry high speed digital signals using a common ground and a ground-sensing indicator.
2. Discussion of the Related Art
Today many homes use sensitive electronic devices as telecommuting and conducting business from one""s home have become more commonplace. In addition to home entertainment centers with expensive audio/visual equipment, many homes now have personal computers, modems, printers, copiers, facsimile machines, telephone answering systems and home security systems. These sensitive electronic devices are connected to the outside world by means of telephone lines (both voice lines and high speed data lines), coaxial transmission lines (both cable TV and satellite dish antennas) and AC power lines.
Lightning is a major source of overvoltage conditions on residential wiring. An overvoltage condition can result from a direct lightning strike induced in AC transmission lines, cables, telephone lines or any conductive path by a nearby lightening strike, or it can be due to correction of the power factor (VI cos"THgr") by electric utility companies when their capacitor banks are switched. It is estimated that there are over 90 million lightning bolts striking the United States annually generating innumerable induced voltage surges and millions of dollars in equipment loss. Each year thousands of homes and businesses are damaged or destroyed by lightning strikes and the surges they produce. For example, in 1990 residential claims for lightning damage exceeded one billion dollars. This number will inevitably increase as homeowners purchase more sophisticated electronics equipment.
Overvoltage conditions can also result from power line crosses caused, for example, by a vehicle striking a utility pole. Transients can also originate within the home when inductive loads such as electric motors are switched. Transients further result from switching non-inductive loads and can be induced on wiring in the home.
Overvoltage conditions can also result from the loss of AC neutral. The loss of AC neutral can result from corrosion of the meter pan at the service entrance where the meter is located outside the building and is exposed to the elements. If that occurs, devices that are normally connected to 110 to 120 volts AC are subjected to 220 to 240 volts AC. Most plug-in secondary surge suppressors in use today are not designed to withstand the loss of AC neutral and may create a fire hazard when AC neutral is lost.
Primary overvoltage protection for telephone lines is provided by surge arresters located in network interface devices (xe2x80x9cNIDsxe2x80x9d) mounted on the outside of the home. See electrical protection devices 73 in FIG. 3 of U.S. Pat. No. 4,979,209 issued to Collins et at. on Dec. 19, 1990. Grounding for these overvoltage protection devices is provided by means of an earth ground brought into the enclosure at the time of installation and attached to ground bus 71 at terminal 71A.
Coaxial transmission lines have been used to carry signals requiring large bandwidth, such as analog video signals and very high speed digital signals. It is known in the prior art to protect coaxial transmission lines from overvoltage conditions using coaxial surge arrestors. One such coaxial surge arrestor is shown in U.S. Pat. No. 5,566,056 issued to Nisar A. Chaudhry on Oct. 15, 1996.
Overvoltage and overcurrent protection for high speed digital data lines requires different surge protection than that for standard voice telephone lines. Surge protection for high speed digital data lines must have low capacitance and low insertion loss to avoid attenuating the high speed data signals. Moreover, surge protection for high speed data lines must protect against both high voltage surges and low voltage surges. An Ethernet overvoltage and overcurrent protection system for 10BASE-T Ethernet and 100BASE-TX Fast Ethernet networks which does not significantly decrease the bandwidth of the networks is shown in U.S. Pat. No. 5,790,363 issued to Nisar A. Chaudhry on Aug. 4, 1998.
Many homeowners attempt to protect their valuable electronic equipment with plug-in surge suppressors. However, such devices do not protect equipment from the large amplitude pulses induced by lightning strikes, although they do offer some protection against low energy transients originating within the home. A surge suppressor is only as good as the ground to which it is connected. A plug-in surge suppressor which protects both AC power lines and telephone lines is shown in U.S. Pat. No. 4,438,477 issued to Cawley on Mar. 20, 1984.
Many plug-in surge suppressors do not have the ability to sense the presence of a ground connection. Thus, in the event of no ground connection, serious damage may occur to electronic equipment connected to the plug-in surge suppressor during surge activity.
Plug-in surge suppressors sold prior to 1998 had to meet the requirements of Underwriters Laboratories Standard 1449, First Edition (xe2x80x9cUL 1449 Ed 1xe2x80x9d) dated Aug. 28, 1985. On Aug. 15, 1996 UL 1449 Second Edition (xe2x80x9cUL 1449 Ed 2xe2x80x9d) was released and became effective Feb. 16, 1998. It requires new tests that were not required by UL 1449 Ed 1. For example, a new test that must be passed to qualify under UL 1449 Ed 2 is that a plug-in surge suppressor must not create a fire hazard in the event AC neutral is lost. Since UL 1449 Ed 2 was promulgated in August 1996, companies have been struggling to comply with the new test requirements.
The present invention provides a power line surge protection device which is more effective in protecting electronic equipment and human beings from the dangers associated with overvoltage conditions such as those induced by lightning, the loss of a ground connection, and the loss of AC neutral, and which meet the more stringent requirements of UL 1449 Ed 2.
The present invention significantly reduces the likelihood that a fire will result from an overvoltage condition or a component failure in a typical power line surge protection device.
The present invention also provides a common ground, which provides a necessary surge diversion path, and which serves as a ground potential equalizer for reducing any ground loop currents that may cause serious damage to electronic equipment connected thereto during surge activity.
The above advantages and features are of representative embodiments only, and are presented only to assist in understanding the invention. It should be understood that they are not to be considered limitations on the invention as defined by the claims, or limitations on equivalents to the claims. For instance, some of these advantages may seem mutually contradictory, in that they cannot be simultaneously implemented in a single embodiment. Similarly, some advantages are primarily applicable to one aspect of the invention. Thus, this summary of features and advantages should not be considered dispositive in determining equivalence. Additional features and advantages of the invention will become apparent in the following description, from the drawings, and from the claims.